1. Field of the Invention
The present invention relates to a multi-frequency antenna, and more particularly, to a multi-frequency antenna for use in a wireless local area network system.
2. Description of the Prior Art
An antenna is utilized to radiate or receive electromagnetic waves for transmission or reception of radio frequency signals. For an electronic product with communications functions of a wireless local area network (WLAN), such as a notebook, there is commonly a built-in antenna utilized to access the WLAN system. With the advance of the wireless communication technologies, various wireless communications systems may adopt different operating frequencies. For example, the wireless LAN standard IEEE 802.11a developed by the Institute of Electrical and Electronics Engineers (IEEE) adopts a central frequency of about 5 GHz, and the evolution of the standard IEEE 802.11, IEEE 802.11b, adopts a central frequency of about 2.4 GHz. Therefore, for the purpose of convenience for users to access a WLAN, an ideal, single antenna should be able to operate for multi-frequency bands utilized by different WLAN systems. In addition, the size of the antenna should be designed as small size as possible to catch up with the tendency of miniaturization in wireless communications facilities.
Please refer to FIG. 1, which is a schematic diagram of an inverted-F planar multi-frequency antenna 10 according to the prior art. The planar multi-frequency antenna 10 includes an interconnecting element 12, a planar radiating element 14 and a planar grounding element 16. The interconnecting element 12 has a connecting terminal 20 coupled to a feeding wire 18, for feeding signals into the planar radiating element 14. The planar radiating element 14 and the planar grounding element 16 generate electromagnetic waves, and thereby a metal bar P1 of the planar radiating element 14 is utilized to radiate higher frequency electromagnetic waves and a metal bar P2 thereof radiates lower frequency electromagnetic waves.
As known well in the art, a conducting path of an antenna is preferred to be longer than or approximate to ¼ wavelength of the radiating wave. With the ¼ wavelength limitation, the planar radiating element 14 mostly occupies a certain planar space such that the planar multi-frequency antenna 10 cannot be reduced in size effectively, which is inadequate for requirements of miniaturization.